Yeah, if you're going to wade into the deep end of online advertising, you need to know a few definitions.

Cost Per Impression Ads (CPM) - Cost per mille (CPM), is a commonly used measurement in advertising. In Latin mille means thousand, therefore, CPM means cost per thousand. Radio, television, newspaper, magazine, Out-of-home advertising and online advertising can be purchased on the basis of what it costs to show the ad to one thousand viewers (CPM). It is used in marketing as a benchmark to calculate the relative cost of an advertising campaign or an ad message in a given medium. Rather than an absolute cost, CPM estimates the cost per 1000 views of the ad. Source - Wikipedia

Most ad campaigns are measured as CPM, even CPC campaigns will get translated into "eCPM" or effective CPM.

Cost Per Click Ads (CPC) - Pay per click (PPC) is an advertising model used on search engines, advertising networks, and content websites/blogs, where advertisers only pay when a user actually clicks on an ad to visit the advertiser's website. Advertisers bid on keywords they predict their target market will use as search terms when they are looking for a product or service. When a user types a keyword query matching the advertiser's keyword list, or views a page with relevant content, the advertiser's ad may be shown. These ads are called a "Sponsored link" or "sponsored ads" and appear next to or above the "natural" or organic results on search engine results pages, or anywhere a webmaster/blogger chooses on a content page. Source - Wikipedia

Google Adsense is the most popular CPC advertiser, but they also provide CPM-centric ads.

Cost Per Aquisition Ads (CPA) - Cost Per Action or CPA (sometimes known as Pay Per Action or PPA) is an online advertising pricing model, where the advertiser pays for each specified action (a purchase, a form submission, and so on) linked to the advertisement. Source - Wikipedia

While keeping these terms in mind, it's also critical to know if the CPM you see for any ad network is gross or net. If it's gross, that's the total CPM including the Ad Network's share. If it's net, it's actually the revenue you've earned. While most ad networks clearly state "net" eCPM, make sure to clarify the point, just in case.

Cheers.